1. Field of the Invention
The present invention relates to a wave front aberration measuring apparatus for use in an optical disk apparatus and a laser beam printer, for example, for obtaining a spot image by focusing a laser beam from a laser source through an imaging lens, and more particularly to an apparatus for measuring an astigmatism and coma aberration.
2. Description of the Prior Art
An optical system of a magneto-optical disk apparatus, for example, is one type optical system as mentioned above. This optical system focuses a laser beam from a laser source upon the disk by an objective optical system, records information on the disk and reproduces information from a disk. A laser flux must be focused on the disk into a spot as small as the diffraction limit or so in order to accurately record and reproduce the information. Additionally, the astigmatism or the coma aberration of the laser beam focused upon the disk must be reduced as much as possible.
When the wave front of the laser beam incident on the disk has an astigmatism, separation results in a focusing position in the vertical plane and a focusing position in the horizontal plane results, and the laser beam cannot be sufficiently focused. Also when the central axis of the laser beam is inclined relative to the optical axis of the objective optical system, a coma aberration will occur and the spot upon the disk will enlarge.
Accordingly, this kind of optical system requires both the measurement of the astigmatism and coma aberration of the beam incident on an object, and correction of each aberration based on the measurement.
Japanese Patent Laid-open Publication No. Sho 61-109105 discloses a conventional correcting method of astigmatism. The method mentioned in the Publication is that the wave front of a light beam having an astigmatism emitted from a laser diode is corrected before incidence on an objective optical system by an anamorphic optical system.
However, an astigmatism generated by optical elements placed between an anamorphic optical system and the disk can not be eliminated by the correction of an astigmatism of light beam through the anamorphic optical system. In order to completely eliminate astigmatism of the beam focused on the disk, the aberration generated by the optical element must be reduced. Since very high accuracy is required in making these optical elements placed between the anamorphic optical system and the disk, the cost of making these elements are great. The flattening of a mirror requires especially high accuracy since a mirror of low flatness generates a large astigmatism.
In a conventional method for measuring the inclination of a central axis of a light beam and an optical axis of an objective optical system to thereby measure a coma aberration, a He-Ne laser beam, for example, is first made incident on an objective optical system perpendicular to an optical disk. The beams reflected by each surface of the lenses comprising an objective optical system interfere with each other. The interference fringes are projected onto a screen, and finally the inclination of the objective optical system is measured.
However in the conventional measuring method of an objective optical system, the inclination of the optical axis of an objective optical system cannot be measured with high accuracy because interference fringes occur by superposing all beams reflected from each surface of the lenses comprising the objective optical system, and the inclination or the dislocation of the axes of the lenses affects the interference fringes. Additionally, since this method requires a He--Ne laser apparatus, etc., the costs are greater than a single-purpose method of measuring inclination.
With regard to the measurement of a coma aberration, a highly accurate analysis can be gained by computerizing an interference fringe using an ZYGO 8100 interferometer (a trade name of ZYGO Co. Ltd.). This method is not only costly, but also cannot gain the real time result of the measurement. Therefore, it has been difficult to adjust the inclination of an objective optical system and at the same time confirm the result of the measurement.